Arene ruthenium(II) complexes formed by dehydrogenation of cyclohexadienes with ruthenium(III) trichloride

Arenedi-µ-chloro-ruthenium complexes RuCl2(arene)2 have been prepared by dehydrogenation of the appropriate cyclohexa-1,3-diene or cyclohexa-1,4-diene with ethanolic ruthenium(III) trichloride. They react with ligands (L) such as pyridine, tertiary phosphines, or tertiary arsines to give monomeric complexes RuCl2(arene)L, which are formally analogous to the arenetricarbonylchromium complexes. Analogous dibromo-, di-iodo-, halogeno-(methyl), and dimethyl derivatives have also been prepared; the latter are thermally unstable and air-sensitive. Reaction of the complexes RuCl2(arene)2(arene = benzene or p-cymene) with water gives binuclear tri-µ-chloro-cationic species Ru2C13(arene)2+, and reaction with silver(I) tetrafluoroborate in acetonitrile gives monomeric Ru(arene)(MeCN)32+. I.r. and n.m.r. data for the new complexes are given. The complexes RuCl2(arene)L(L = PBun3 or PPh3) undergo partial or complete arene exchange on heating or on u.v. irradiation in an aromatic solvent, co-ordinated p-cymene being the most easily displaced. The exchange method can be used to prepare hexamethylbenzene complexes, e.g.RuCl2(C6Me6)(PBun3). The results are compared with available data on arene exchange in arenetricarbonylchromium complexes and discussed in terms of electronic and steric effects on metal–arene bonding.